io6 GEOGRAPHICAL DISTRIBUTION 



The Cetacea are not limited to the ocean, or even to salt water, 

 some entering large rivers for considerable distances, and others 

 being exclusively fluviatile. One species of Platanista is extensively 

 distributed throughout nearly the whole of the river systems of the 

 Ganges, Brahmaputra, and Indus, ascending as high as there is 

 water enough to swim in, but apparently never passing out to sea. 

 The individuals inhabiting the Indus and the Ganges must therefore 

 have been for long ages isolated without developing any definite 

 distinguishing anatomical characters ; for those by which the sup- 

 posed P. indi was formerly separated from P. gangetica have been 

 shown by Anderson to be of no constant value. Orcella fluminalis 

 appears to be limited to the Irawaddy river, and at least two distinct 

 species of Dolphin belonging to different genera are found in the 

 waters of the upper Amazon. A Neomeris has been found in the 

 great Chinese river, the Yang-tsi-Kiang, nearly a thousand miles 

 from the sea. It is remarkable, however, that none of the great 

 lakes or inland seas of the world are, according to our present 

 knowledge, inhabited by Cetaceans. A regular seasonal migration 

 has been observed in many of the oceanic Cetacea, especially those 

 inhabiting the North Atlantic, but further observations upon this 

 subject are still much needed. 



The great difference in the manner of life of the Sirenia, as 

 compared with that of the Cetacea, causes a corresponding difference 

 in their geographical distribution. Slow in their movements, and 

 feeding exclusively upon vegetable substances, water-grasses, or fuci, 

 the Sirenia are confined to rivers, estuaries, or coasts where these 

 grow, and are not denizens of the open sea, although of course there 

 is a possibility of accidental transport by the assistance of oceanic 

 currents across considerable distances. Of the three genera exist- 

 ing within historic times, one (Manatus) is exclusively confined to 

 the shores of the tropical Atlantic and the rivers entering into it, 

 individuals scarcely specifically distinguishable being found both on 

 the American and the African side of the ocean. The Dugong 

 (Halicore) is distributed in different colonies, at present isolated, 

 throughout the Indian Ocean from Arabia to North Australia. 

 The Rhytina or Northern Sea-Cow was, for some time before its 

 extinction, limited to a single island in the extreme north of the 

 Pacific Ocean. 



The Pinnipeds, although capable of traversing long reaches of 

 ocean, are less truly aquatic than the last two groups, always 

 resorting to the land or to extensive ice-floes for the purpose of 

 breeding. The geographical range of the various species is generally 

 more or less restricted, usually according to climate, as they are 

 mostly inhabitants either of the Arctic or Antarctic seas and adjacent 

 temperate regions, very few being found within the tropics. For this 

 reason the northern and the southern species are for the most part 



